1. Field of the Invention
This invention relates to a dynamic channel allocation method in a cellular mobile telephone system.
2. Description of the Related Art
In a radiocommunications system of large capacity such as a mobile telephone system, the service area is covered by a plurality of base stations and channels with the same frequency are used by the base stations among which no interference disturbance occurs in order to achieve effective use of frequencies. The mobile telephone system just described is called a cellular system.
Allocation of channels to be used by base stations is roughly divided into two methods. According to one of the methods, channels to be used by each base station are allocated fixedly in advance so that no interference disturbance may occur as a result of predicting the propagation characteristics. This method is called fixed channel allocation and is adopted in present-day mobile telephone system. The other method is called dynamic channel allocation in which a channel which does not cause interference disturbance is selectively used for each communication. While this method requires a complicated apparatus construction, since it allows free use of any channel so long as it does not cause interference disturbance, it is advantageous in that the number of subscribers which can be accommodated is greater than that of the fixed channel allocation. Accordingly, adopting the dynamic channel allocation for a mobile telephone system is undergoing research.
For the dynamic channel allocation method, an autonomous reuse partitioning (hereinafter referred to simply as ARP) method to realize channel allocation with a highly efficient utility of frequency and very simple control has been proposed as disclosed, for example, in a paper published under the title of Autonomous Reuse Partitioning in Cellular Systems, Proceedings of IEEE Vehicular Technology Society, 42th VTS Conference, pp. 782-785, May, 1992.
In the ARP system, channels are selected in accordance with the same of channel number order at all cells, and put into use from a channel which presents a carrier-to-interference ratio (hereinafter referred to as CIR) higher than the required value in both reverse-link (mobile station to base station) and forwards-link (base station to mobile station).
FIG. 1 is a flow chart illustrating control of a base station to which the conventional ARP system is applied.
It is assumed that n channels numbered 1 to n are available at each base station, and that each base station periodically receives and stores interference wave levels U.sub.up (i) of available free channels where i represents the channel number from 1 to n. Further, it is assumed that the transmission power level (hereinafter abbreviated to P.sub.MS) of the mobile station and the transmission power level of the base station (hereinafter abbreviated to P.sub.BS) are known.
When a call request is occurred, the base station stores a receive level of a request for call origination signal (when the call is originated from the mobile station) received through a control channel, or a call response signal (when a mobile station is called) to the call from the mobile station as reverse-link carrier level D.sub.up (step 1300).
In the following description, steps are abbreviated to S so that step 1300 is represented as S1300.
The value obtained by subtracting D.sub.up from P.sub.MS is assumed as the propagation loss (hereinafter referred to as L in abbreviation) between the base station and the mobile station (S1301).
Since the loss level (L) of the radio propagation of reverse-link and forward-link is reversible, the forward-link carrier level (D.sub.down) at the mobile station can be determined by subtracting L from P.sub.BS (S1302).
Assuming channel number to 1 (S1303), the value obtained by subtracting the reverse-link interference wave level of channel 1 (U.sub.up (1)) from D.sub.up, that is, the reverse-link CIR is compared with a required value (hereinafter abbreviated to CIR.sub.th) (S1304).
When the reverse-link CIR is equal to or higher than CIR.sub.th, the base station instructs the mobile station to measure the forward-link interference wave level of channel 1(U.sub.down (1)) and receives the result of measurement from the mobile station (S1305).
The base station then compares the value obtained by subtracting U.sub.down (1) from D.sub.down, that is, the reverse-link CIR, with CIR.sub.th (S1306).
As a result, if the forward-link CIR also is equal to or higher than CIR.sub.th, then the base station allocates channel 1 to the call request (S1307).
On the contrary, when the reverse-link CIR or the forward-link CIR of channel 1 is lower than CIR.sub.th, the base station increments channel number i by one to select next channel 2 (S1309), and thereafter, the processes from S1304 to S1306 are repeated in a similar manner to determine the interference condition.
When the determination for final channel n (S1308) proves that no available channel has been found, the call is blocked (S1310).
By this procedure, a channel having a higher priority degree, that is, a channel whose channel number is closer to 1, presents a higher interference wave level and is allocated to a mobile station closer to a base station which presents a higher D.sub.up. On the other hand, since a channel having a lower priority degree presents a lower interference wave level, it is allocated to a mobile station closer to the boundary of the cell which presents a lower D.sub.up.
FIGS. 14(A) to 14(D) are diagrammatic views illustrating the relationship between the base stations and the mobile stations of channels 4 to 1 when the channel allocation method illustrated in FIG. 1 is applied.
Base stations 3A to 3E have cells 5A to 5E, respectively as service areas, and channel 1 is a channel having the highest priority degree and is preferentially allocated when, for example, as seen in FIG. 14(D), a mobile station is present in cell 5A in mobile station presence area 4A which is within radius R1 from base station 3A. In this instance, also in base station 3B adjacent to base station 3A, the identical channel is allocated for communication with a mobile station within mobile station presence area 4B within radius R1 from base station 3B and used simultaneously.
Meanwhile, when a mobile station positioned in cell 5A is present within mobile station presence area 4A between radius R1 and radius R2 from base station 3A as shown in FIG. 14(C), channel 2 which has the second highest priority order is allocated to the mobile station. In this instance, for example, also in base station 3C (located farther than base station 3B from base station 3A) having cell 5C, identical channel 2 is simultaneously allocated for communication between a mobile station present within mobile station presence area 4C within radius R2 from base station 3C.
This similarly applies to the other channels, and if channel 4 has the lowest priority degree, when mobile station presence area 4A is in the proximity of radius R4 from base station 3A which is in the proximity of the outermost circumference of cell 5A, channel 4 is allocated to a mobile station within cell 5A as seen in FIG. 14(A).
In this instance, also for base station 3E located farther from base station 3A, if a mobile station is present in the proximity of the outermost circumference of cell 5E of base station 3E, channel 4 is used for communication between the mobile station and base station 3E.
In this manner, as long as the allocation is realized in accordance with the same order at each base station, the distance between one base station and one mobile station is automatically leveled to an approximately equal value for each channel, and individual channels are allocated at minimum necessary distances such as D1 to D4 for simultaneous use (hereinafter referred as reuse) corresponding to the distances (R1 to R4) between the base stations and the mobile stations as seen in FIG. 14. As a result, the average reuse distance is reduced compared with that of fixed channel allocation. Consequently, a greater number of subscribers can be accommodated in each service area.
In the conventional channel allocation method of the ARP method for a mobile communications system described above, transmission power control is performed.
In the general transmission power control method, the transmission output power of the transmission side is controlled so that the carrier level on the reception side can be kept at a desired value. The desired value of the carrier level is set to a minimum value at which no quality deterioration by noise is caused. If the desired value is set in this manner, when a mobile station as a terminal is positioned closer to a base station, the transmission power can be suppressed, accordingly, the consumption of batteries provided at the mobile station can be reduced and the available service time can be increased.
If transmission power control is performed in this manner, whether a mobile station is positioned closer to or far from a base station, the carrier level is substantially fixed. Accordingly, if the algorithm of the ARP method illustrated in FIG. 1 is used as is, then it is difficult for a plurality of mobile stations whose distances to a base station are equal to reuse the same channel within the same cell in FIG. 14.
As a result, there is a drawback in that the frequency of use of channels of high selection priority degrees is not high and the traffic accommodation capacity is low.